1. Field of the Invention
The present invention relates generally to a power amplifier for an audio signal, and more particularly to a semiconductor integrated circuit including a high-efficiency power amplifier that amplifies audio signals of a plurality of channels with high efficiency, for example, a power amplifier circuit suitably applicable to a monolithic power amplifier IC mounted in a car stereo apparatus.
2. Description of the Related Art
In recent years, there has been a trend that car audio apparatuses have more power, multi-channel configurations and multimedia capabilities. With this trend, there has been an increasing demand for a power amplifier with high efficiency and low heat production. Conventionally, various attempts have been made to realize an audio power amplifier system with lower heat production (i.e. lower power consumption) while keeping the level of output power.
For instance, Jpn. Pat. Appln. KOKAI Publication No. 2002-353746 discloses, as an example of an analog-type high-efficiency amplifier system, an amplifier system of a scheme called “keyed B class (KB-class)”, wherein a plurality of power amplifiers are vertically stacked between a plurality of power supply rails. In this scheme, for example, two bridge tied load (BTL) amplifiers, each including an output bridge circuit in which output transistors are bridge-connected, are vertically stacked and used. When a power supply voltage is Vcc, a push-pull operation is executed with a Vcc/2 power supply at a time when a small signal is input. On the other hand, when a large signal is input, the push-pull operation is switched to a conventional class AB operation using a Vcc power supply. Thereby, a power amplifier circuit with high efficiency and low heat production is realized (see FIG. 2 to FIG. 4 of the above-mentioned KOKAI 2002-353746).
As will be described later in detail, a conventional KB-class amplifier comprises an output transistor Q1 that is connected to a first power supply rail (power supply line), an output transistor Q2 that is connected to a second power supply rail, and an output transistor Q3 that is connected to a third power supply rail. The output transistors Q1 and Q2 are switched, whereby a push-pull operation and a class AB operation are switched. The KB-class amplifier further comprises potential detection/driving control transistors for controlling the operation states of the transistors Q1 and Q2, an input circuit for delivering an input signal, a driving circuit for amplifying the signal from the input circuit and driving the output transistor Q3 at the rear stage, and a feedback circuit for feeding the signal at an output terminal back to the input circuit.
This type of KB-class amplifier, however, has a problem in that switching of power supply from a plurality of different power supply rails has to be executed in a low output power state, and a sufficiently high efficiency cannot be exhibited. To avoid this, there arises another problem that the chip area of the transistor Q2 has to be increased to sufficiently enhance the load driving performance of the transistor Q2.
Under the circumstances, there has been a demand for a power amplifier system that can amplify audio signals of plural channels with high efficiency, while the amount of heat produced is small. To be more specific, there has been a demand for a circuit scheme for a power amplifier system using a plurality of power supply rails and a plurality of output transistors, wherein smooth switching can be effected between the output transistors and the high-efficiency function can be exhibited to the maximum, without unnecessarily increasing the chip area of the power transistors.